Lead Gardens has an illustrated tutorial on making a “classic” wargames building of the sort you see in old issue of The Courier, or in one of Charles Grant’s publications.
As it turns out, Birch Seeds, when taken apart, make amazing scale model leaves for basing and other terrain projects. it reminds me of something John Hill, author of Johnny Reb and Squad Leader once said to me about wargames modeling: Nature reproduces itself in miniature.
The Daddy’s Little Men Blog has a brief article on his experience trying to make an orange grove for a game set in southern Lebanon.
Dr. Merkury’s Lab offers some templates for making 28mm scale modern style buildings. Print them out, and then use them as guidelines to cut your walls from foam core.